1. Field of the Invention
The present invention relates to a process for preparing monohydrated sodium phenylpyruvate.
2. Description of the Prior Art
Phenylpyruvic acid or the salt thereof is a useful raw material for preparing phenylalanine. Phenylpyruvic acid can be prepared by acid hydrolysis of .alpha.-acetamidocinnamic acid (Herbst, R. M. and Shemin, D. (1943) Org. Syn. Coil. 3, 519) or by biscarbonylation of benzylchloride (General Director of Agency of Industrial Science and Technology and Nissan Chemical Industries, Ltd., Japan (1987) U.S. Pat. No. 4,689,431; Ethyl Corporation (1985) U.S. Pat. No. 4,492,798; Montedison S.p.A. (1982) U.S. Pat. No. 4,351,952; Rhone-Poulenc Industries (1979) U.S. Pat. No. 4,152,352). However, phenylpyruvic acid is not a stable compound at room temperature. Instability causes storage and transportation problems.
U.S. Pat. No. 4,518,800 issued to Schouteeten et al. on May 21, 1985 discloses that monohydrated sodium phenylpyruvate, which is a stable compound, can be prepared by reacting benzaldehyde with the stoichiometric quantity of hydantoin in an aqueous medium in the presence of a catalytic quantity of ethanolamine at high temperature, treating the reaction mixture with an excess of sodium hydroxide, acidifying the obtained solution to pH 9 with concentrated hydrochloric acid, and washing the formed precipitate with iced water. However, there are two main disadvantages in the above-mentioned process for the preparation of monohydrated sodium phenylpyruvate. First, much excess sodium hydroxide is used. This subsequently leads to an increase in the amount of hydrochloric acid used. Second, washing monohydrated sodium phenylpyruvate with iced water substantially reduces the yield, since monohydrated sodium phenylpyruvate is soluble in water. To check the yield of monohydrated sodium phenylpyruvate, the inventors followed the process as described in U.S. Pat. No. 4,518,800, and found that the first isolation yield of monohydrated sodium phenylpyruvate was about 52% of the theoretical value instead of 75% and the overall yield was about 84% instead of 99%, based on HPLC analysis. The low yield in the first isolation is due to the washing of the product with iced water as monohydrated sodium phenylpyruvate is soluble in iced water. The overestimated overall yield (99%) in U.S. Pat. No. 4,518,800 is a result of the presence of large portion of sodium chloride in the second isolation products. The present invention provides a more economic process for the preparation of monohydrated sodium phenylpyruvate in high yield.